Natural gas is increasingly being used as a source of energy by both residential and commercial consumers due to its convenience, cleanliness and efficiency. Natural gas can be used for heating, cooling and cooking, and is quickly replacing oil and coal as the preferred energy source in many parts of the country. An important reason for its growth in popularity lies in the ease of delivery in which gas can be provided to a consumer. Instead of relying on periodic deliveries which can be hindered by inclimate weather, gas is typically piped directly to the consumer. Most new communities are being constructed with the exclusive use of natural gas in mind, by installing extensive gas line networks to serve the current and future residents.
Despite the convenience of natural gas, it also presents a significant danger. Since natural gas is highly flammable, a rupture to the gas lines resulting in gas leakage can cause catastrophic damage to adjacent structures due to fires or explosions. This danger is most acute in areas of the country which experience periodic earthquakes or other seismic disturbances that cause the buried pipelines to rupture. For example, a large percentage of homes destroyed during recent earthquakes in California were consumed by fires started by leakage from ruptured gas lines. Earthquake preparedness experts caution all natural gas consumers to promptly close off all gas lines entering a structure shortly after experiencing an earthquake. By closing off these gas lines, the amount of gas which could leak into the structure can be minimized, and the consequent risk of fire reduced.
Automatic shut-off valves which close off the gas line in response to a seismic disturbance are well known in the art. A common attribute of many such automatic shut off valves is the use of a ball which can be dislodged by a seismic disturbance and which falls into a valve seat position, thereby blocking flow of a gas through the valve structure. Examples of these prior art valves include U.S. Pat. No. 3,747,616, by Lloyd, U.S. Pat. No. 4,212,313, by Winters, and U.S. Pat. No. 4,331,171, by Novi. Each of these patents disclose an initially stationary ball placed upon a pedestal which becomes dislodged from the pedestal by the vibration caused by the seismic disturbance.
These pedestal type valves proved to be unreliable, since virtually any vibration to the valve structure would cause the ball to lodge in the valve seat. Consequently, valves were developed which allowed for limited movement of the ball within the valve structure. Examples of this type of prior art valve include U.S. Pat. No. 4,485 832 by Plemmons et al., and U.S. Pat. No. 4,565,208, by Ritchie et al. These two patents each show valves having a central chamber joined by an inlet and an outlet aperture with a valve sea provided therebetween. A circular track is located above the valve chamber having an obstruction blocking a portion of the track. Upon the track are placed one or more balls which are free to roll along the circumference of the track. Upon the event of a shock or disturbance, a ball can either leave the track and fall directly onto the valve seat, or rebound off the obstruction into the valve seat. Once seated, the ball completely blocks the flow of gas between the inlet and the outlet aperture, effectively closing the valve. Both patents also disclose a resetting mechanism in which a rod extends upward through the valve chamber to dislodge the ball from contact with the valve seat and return it to the initial position on the track.
Although these prior art valves are able to cut off the flow of gas in the event of a seismic disturbance more reliably than the pedestal type valves, they also have several significant drawbacks which render them impractical for ordinary usage. The most significant problem with the prior art valves is unpredictability. The obstruction disposed in the track above the valve chamber of each of the valves essentially comprises a wall which the ball rebounds against. The Plemmons obstruction is more severe, comprising a substantially perpendicular surface relative the floor of the track. A jostling motion effecting the valve and causing the ball to rebound off the wall of the obstruction could cause the ball to either fall into the valve seat or to rebound back along the track. The direction of the rebound would depend more upon the direction of the origin of the seismic disturbance, rather than the size or magnitude of the disturbance.
Similarly, the Ritchie valve has a bell-shaped obstruction. Although more rounded than in the Plemmons obstruction, the vertical component of the Ritchie obstruction is very steep. As in the Plemmons valve, upon striking the obstruction, the direction of rebound of the ball is unpredictable. During ordinary operation, this rebound effect results in a greater number of false alarms, or situations in which the valve is shut-off due to vibrations caused by movement of trucks nearby or other routine disturbances, rather than that caused by a true earthquake. These false alarms significantly inconvenience natural gas consumers, requiring them to be without the resource until a maintenance engineer can reset the valves. In areas of high traffic, the false alarm rate can rise to as much as several false triggers per day.
A secondary problem with the Plemmons and Ritchie valves is that of gas leakage past the resetting mechanism. The hand actuated rod of each prior art valve has a handle or knob which is accessible externally of the valve body, so that an operator can easily return the ball to the initial position. However, by virtue of the fact that the handle extends externally of the valve casing, an opening is provided which enables a path for the leakage of gas.
Both Plemmons and Ritchie attempt to remedy this problem by including O-ring seals surrounding a portion of the rod. These O-rings are intended to provide a seal between the rod and the sleeve which carries the rod. However, dust or other airborne particulate matter can enter the valve body through the handle opening, and settle upon the rod, sleeve and O-ring. The particulate build-up tends to prevent the O-ring from forming a positive seal between the rod and sleeve. This problem is compounded when the rod fails to return fully to the bottom of the valve chamber. With the rod partially extended, the O-ring may not be in a position to form a seal between the rod and the chamber, allowing additional leakage. Without a positive seal between the rod and the sleeve, gas can leak out of the valve chamber. Even trace amounts of gas leakage can present a substantial hazard to a structure. The American National Standards Institute has recognized the severity of this problem, and levied ANSI Z21.70-1981 which requires "pins, stems or other linkage passing through the valve body or casing" to be sealed to provide gastight construction.
An additional problem with the prior art valves is that of gas turbulence within the valve chamber. The high flow rate of gas moving through the valve structure form eddy currents within the valve chamber. Rather than flowing directly between the inlet and outlet apertures, the gas tends to draw upward into the track portion of the valve. The internal turbulence within the valve casing reduces the overall efficiency of the valve, resulting in reduced effective gas line pressure as measured at the outlet aperture. These eddy currents within the valve casing also contribute to the unpredictability of the valve cut-off.
Thus, it would be desirable to provide a gas shut off valve responsive to earthquakes or other seismic disturbances having increased predictability and reliability. It would also be desirable to provide a gas shut-off valve which is fully enclosed so as to prevent any gas leakage, and would satisfy the requirements of the ANSI standard. It would be further desirable to provide a gas shut-off valve having a more efficient flow rate and reduced internal turbulence.